In general, bearing steel is made in a converter or an electric furnace and then refined in a ladle by allowing a strong reducing atmosphere to be maintained to decrease an amount of non-metallic inclusions therein, and continuously refined in a state in which an oxygen content (T[O]) is decreased up to 12 ppm or less through a vacuum degassing process. Subsequently, the bearing steel is solidified into slabs or steel ingots through a casting process and then rolled into billets after soaking is performed in order to remove segregation and large carbides contained in the material. Thereafter, the bearing steel billets are formed as wires or bars by performing an extremely slow cooling operation in a rolling mill for softening the material. The wires or bars are machined into balls or rollers, rolling elements of the bearing steel, or inner and outer races through spheroidizing annealing and subsequently, quenching and tempering treatments are performed as hardening heat treatments to produce bearings, final products, through a polishing process.
With respect to the bearing steel produced through a casting process as above, it is generally considered that the generation of segregation and large carbides in the material may be unavoidable, due to of high carbon and high chromium contents. That is, since a difference between solubilities of a solute element in a solid phase and a liquid phase exists during solidification, solute atoms may be discharged and accumulated at a forward edge of a solid-liquid interface, and this may subsequently generate microsegregation between dendrites. The microsegregation between dendrites is absorbed into the inside of a solidification shrinkage cavity generated in a center portion of the material at the completion of the solidification to thus cause a large amount of center segregation, and as a result, large carbides may be generated in a center segregation band of the material. These large carbides may become a cause of a premature fatigue fracture initiated in the region of the large carbides during a fatigue test or in actual use to induce a bearing flaking phenomenon. FIG. 1 illustrates large carbides generated in a shrinkage cavity and it may be confirmed that a portion of the shrinkage cavity may not be filled.
A prior art method for removing large carbides in a segregation band of a casting material most adversely affecting mechanical properties of bearing steel may include a method of preventing the absorption of microsegregation into a shrinkage cavity by casting under a low pressure and a method of removing center segregation and large carbides by diffusion through soaking at a high temperature of 1000° C. or more after casting.
Also, numerous attempts to control the occurrence of segregation by facility have been made, for example, a method of reducing segregation in bearing steel by performing a continuous vertical casting operation under a low pressure ranging from 10 mm to 100 mm has been proposed in Japanese Patent Application Laid-Open Publication No. 1996-132205 and a technique of installing a roll in a solidification portion and performing under a low pressure to control segregation has been proposed in Japanese Patent Application Laid-Open Publication No. 1994-248302.
A technique of rolling a slab under a low pressure and removing large carbides by soaking at a temperature ranging from 1150° C. to 1250° C. for 2 hours to 5 hours before blooming has been proposed in Japanese Patent Application Laid-Open Publication No. 1995-299550, a technique of inhibiting large carbides by using steel having a phosphorus (P) concentration ranging from 0.002 wt % to 0.009 wt % and maintaining the steel at a temperature ranging from 1150° C. to 1260° C. for less than 2 hours has been proposed in Japanese Patent Application Laid-Open Publication No. 2006-016683, and a technique related to high-carbon chromium bearing steel, in which large carbides are decreased and diffusion annihilated by maintaining the steel at a high temperature of 1050° C. or more for 1 hour to 4 hours, has been proposed in Japanese Patent Application Laid-Open Publication No. 2009-127113.
However, since it may be impossible to completely prevent the generation of segregation and large carbides by using a current technique of casting under a low pressure, soaking must be undertaken after the casting. In addition, soaking by maintaining a cast material at about 1150° C. or more, a generation temperature of carbides, in order to inhibit the formation of large carbides may not only largely increase energy consumption, but decarburization may also excessively occur in a surface layer of the material. Thus, hot scarfing may be required before billet rolling and as a result, yield may also be decreased.
Therefore, since large carbides in a segregation band of a bearing steel casting material may be generated by the growth of microsegregation between dendrites into segregation, a technique of fundamentally addressing the foregoing limitations may be required.